In Fig. REF , we show in white contours the spatial distribution of the HCN \(J\) =3–2 velocity-integrated emission between \(-20\)  and \(+5\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  over the southern outflow lobe. The emission
draws two extended features of different Parallatic Angles, which coincide with the two CO outflow cavities identified by [1]}. The emission peaks at the nominal position of the outflow shock position B1. Secondary emission peaks are found towards the shock positions B0 and B2, at about 30 north and south of B1, respectively [2]}. Hardly any emission is detected along the outflow between B0 and the protostar (located North, red cross in Fig. 1).
The systemic velocity of L1157 is \(+2.7\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  [3]}. We show in color scale in Fig. REF  the HCN \(J\) =3–2 emission integrated over the three velocity intervals (in \(\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\) ): [\(-\) 10.2; \(-\) 2.7], [\(-2.7\) ; \(+\) 4.3] and [\(+\) 4.3; \(+\) 6.3], from left to right. We find that the low-velocity emission (middle panel in Fig. REF ) is associated with the bright shocked regions B0, B1, and B2, as well as faint and extended emission tracing the whole outflow lobe. As shown by [4]}, the outflow is oriented almost in the plane of the sky
(inclination \(i\sim 73\) ). We propose that the HCN emission between \(-2.7\)  and \(+4.3\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  could arise from low-velocity shocks associated with the cavity walls of the outflow, which would explain the extended emission at blue- and red-shifted velocities.
This is consistent with the distribution of the HCN \(J\) =1–0 emission line as mapped by [5]} at \(\approx 5\)  with the Plateau de Bure Interferometer.
The high-velocity blueshifted gas traces the young shocks B0 and B1 (left panel in Fig. REF ), while the redshifted emission is only detected toward the oldest B2 shock (right panel). These maps indicate that the shocked gas in B0 and B1 emit at a velocity distinct from that of B2. Interestingly, the brightest HCN \(J\) =3–2 emission coincides with the apex of the outflow cavities, consistent with the high angular resolution NOEMA images of H\(^{13}\) CN and HC\(^{15}\) N [6]}, [7]}.
